The influence of supra-optimal root-zone temperatures on growth and stomatal conductance in Capsicum annuum L.

Pepper (Capsicum annuum L.) plants were grown aeroponically in a Singapore greenhouse under natural diurnally fluctuating ambient shoot temperatures, but at two different root-zone temperatures (RZTs): a constant 20 +/- 2 deg rees C RZT and a diurnally fluctuating ambient (A) (25-40 degrees C) RZT, P lants grown at 20-RZT had more leaves, greater leaf area and dry weight tha n A-RZT plants. Reciprocal transfer experiments were conducted between RZTs to investigate the effect on plant growth, stomatal conductance (g(s)) and water relations. Transfer of plants from A-RZT to 20-RZT increased plant d ry weight, leaf area, number of leaves, shoot water potential (Psi(shoot)), and g(s); while transfer of plants from 20-RZT to A-RZT decreased these pa rameters. Root hydraulic conductivity was measured in the latter transfer a nd decreased by 80% after 23 d at A-RZT. Transfer of plants from 20-RZT to A-RZT had no effect on xylem ABA concentration or xylem nitrate concentrati on, but reduced xylem sap pH by 0.2 units. At both RZTs, g(s) measured in t he youngest fully expanded leaves increased with plant development. In plan ts with the same number of leaves, A-RZT plants had a higher g(s) than 20-R ZT plants, but only under high atmospheric vapour pressure deficit. The rol es of chemical signals and hydraulic factors in controlling g(s) of aeropon ically grown Capsicum plants at different RZTs are discussed.